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Has anyone installed a Curt Class 3 hitch on a GLK Bluetec? I've got the thing up in place, and in the process of torquing the bolts that clamp around the bumper supports, I got that feel that either the bolts were stretching or the supports were being crushed, where the torque to turn the bolts farther went down instead of up. The bumper supports have raised stampings on the top surface, and it could have been them flattening out, but I decided to stop. They are torqued to 60 ft-lb instead of 75. Also, the forward facing bolts that clamp around the frame member behind the differential are supposed to be torqued to 110 ft lbs. I stopped at 45 because the upper left bolt was about to contact the differential cover.
Any thoughts? I contacted Curt and they said "take it to an installer". I'm a retired aircraft mechanic with a recently calibrated torque wrench..
This is the hitch in question. The forward mount has the same arrangement as the two side ones: a heavy plate and long bolts..
Last edited by John CC; Sep 17, 2021 at 11:55 AM.
Reason: Fixed link to point to the right hitch
I installed the same on mine last April. Knowing that the bolts near the differential would be very close (after watching an online video of such), I purchased a few bolts that were 1/2 inch shorter. That helped give a little more clearance to the top two. I too had a problem torquing them to the recommended specs. I couldn't get them near to 110 ft lbs, no matter how hard I pulled on it with my whole body. I was too tired of fighting the damn thing, and just said enough is enough, it's tighter than any hitch I ever attached.... mine will be used almost solely for a bike rack, so not as concerned as I would be if I were to pull a camper or something.
Well, I spent most of the afternoon fussing with it. I took the forward attachment apart and the cross member was definitely starting to collapse. There is no way you could ever get those bolts to 110 ft-lbs without crushing it. I think it's worse this time around, so I stopped at 40 ft-lbs. I wonder if they got the torque recommendation out of an engineering manual somewhere.
I also replaced the 2-3/4" bolts on the top with 2-1/2" bolts to get a little better clearance at the differential cover. I think I lost about half of that 1/4" to further collapsing of the cross member. I calculated the new bolts would just clear the end of the threads in the nut plate but when it was done about three threads were protruding.
Crappy cell phone picture but the gap between the cross member and the rule from 3" to 7" is from compression. If you look closely at the face of the member you can see where it is starting to bulge out.
I installed the same on mine last April. Knowing that the bolts near the differential would be very close (after watching an online video of such), I purchased a few bolts that were 1/2 inch shorter. That helped give a little more clearance to the top two. I too had a problem torquing them to the recommended specs. I couldn't get them near to 110 ft lbs, no matter how hard I pulled on it with my whole body. I was too tired of fighting the damn thing, and just said enough is enough, it's tighter than any hitch I ever attached.... mine will be used almost solely for a bike rack, so not as concerned as I would be if I were to pull a camper or something.
I'm not sure where you guys are getting the torque numbers from, but I just downloaded / read the pdf instructions from the link in the op's post (see attached) and it says:
Torque all 1/2- 13 hex bolts to 75 lb-ft.
Torque all 7/16" hardware to 59 lb-ft.
I'm thinking of getting this hitch but the 110 ft-lb sounds crazy high. Were these numbers from the instructions that came with the hitch?
Well, that's interesting. Here's the instructions I downloaded a few weeks ago. You'll note that step 13 is different.
While the one you posted is more reasonable, those forward facing bolts were originally torqued to 45 ft-lbs and the cross member had already started to deform.
Hmm... File must be too big... I'll try to post one page.
The printed instructions in the box agreed with the ones I'm posting here, too.
(Later that same day) I've added Page 6 from my downloaded instructions. There is a different document number (13155 vs. 13177) and date (10/28/20 vs. 4/20/21). I downloaded the instructions after 4/21 as I didn't even have the car until mid July, and they are the same as the printed copy in the box, which was shipped to me immediately after it was received from Curt, so Curt shipped the hitch with those directions sometime after July 1. Also, when I called Curt, I specifically asked about the 110 number and they made no mention of updated instructions. (So, their customer support was not too helpful...)
The hitch itself seems well designed and constructed. I wouldn't hesitate to buy it again, but I would probably go with 60 ft-lbs on the vertical 1/2" bolts, 45 on the horizontal ones, and 60 on the two 7/16" bolts. Be sure to leave the 7/16" bolts and the bolts on the bumper supports loose while you torque the horizontal 1/2" bolts So it can center itself on the frame cross member..
Thanks for the extra info, 45 and 60 sounds plenty good and tight to me. I'm not going to be hauling anything that's even close to the limit of the hitch, I'm sure it'll be fine. 👍
13177 is a different hitch, and fits the GLK350. I can't tell from the pictures or the instructions what the difference is, but I'm guessing it has to do with the shape of the well for the spare tire/Bluetec tank. Anyhow, 13155 is the one that fits the 250, and is the right one for my car, and still has instructions referencing 110 ft-lbs.
There is a Draw-Tite hitch for the GLK350 that uses some spacers in the cross member to prevent crushing. I'm not sure which is better, spacers and tight bolts or looser bolts. The Draw-Tite transfers the load to the welded flange of the cross member. Not sure if that is any better; I'm thinking sharing the load would be best. (Draw-Tite instructions attached.)
I think the construction of the Curt hitch is much more robust, but if you crush the cross member then all bets are off.
OK, I just figured out I posted a link to the GLK350 hitch, not the GLK250 hitch in post #1. I went back and corrected the link for posterity. Both hitches look identical in the photos. So, back to square one; the torques in the instructions are not suitable.
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